The present invention relates to metal coating particularly for plastic and other dielectric articles.
Electroless or autocatalytic deposition of metallic coatings on non-conductive or dielectric surfaces, such as plastics, is a well known process with broad applications in diversified industries such as automotive, radio, television, plumbing, marine, home appliance, etc. However, for the benefit of clarity to the non-artisan, it is stated here electroless plating which is the subject of this invention and referred to frequently throughout this disclosure requires no electricity for metallic deposition while electroplating or electrodeposition requires its employment. The field is generally described in my book, "Metallization of Plastics" (Reinhold, New York 1960), see also the U.S. patents cited below.
Heretofore, the application of the electroless coating to surfaces of substrates [following pretreatment of said surfaces] was followed by the commonly used electroplating or electrodeposition process known to the industry particularly electroplated copper, nickel, chromium, gold, silver, etc., to achieve bright surfaces for decorative or functional purposes. These prior art processes involve chemical etching of the plastic substrate mainly to improve adherence of metal thereto, then sensitizing and activating this etched surface, applying a conductive film of silver, copper or nickel and electroplating of the desired metal upon this conductive film.
Recent improvements have eliminated the two-step treatment [i.e., 1--sensitizing and 2--activating] and replaced it with a one-step pretreatment involving colloidal solutions containing non-noble copper ions or noble palladium ions; then a conductive film of copper or nickel is formed and, finally, electroplating metal is applied.
As an example of the non-noble catalyst treatment referred to, the U.S. Pat. No. 4,020,197 issued to Steffen (Apr. 26, 1977) treats the plastic surface in a solution of cuprous chloride, acidified with 10-15% hydrochloric acid, followed by hydrolysis of the cuprous (1) compound to a hydrous oxide in hot water; then acceleration in an alkaline solution of sodium borohydride. Innumerable patents issued to Feldstein such as U.S. Pat. Nos. 3,993,799, 3,993,848, 3,993,801, 3,993,491, 3,993,799 and 3,982,054 also practice the employment of the so-called non-noble metal catalyst for treatment of non-conductive surfaces.
As an example of the noble catalyst referred to, a patent was issued to Shipley, U.S. Pat. No. 3,011,920 (Dec. 5, 1961) in which the noble metal, e.g., palladium, is applied directly to the dielectric surface in the form of a colloidal solution containing the metal in zero valence state. Normally, the metal is formed in the colloidal solution by introducing both palladium chloride and stannous chloride into a highly acidified (with hydrochloric acid) aqueous solution, the palladium ions being reduced within the solution to this zero valence state by the stannous ions. U.S. Pat. Nos. 3,672,938 issued June 27, 1972, to Zeblisky and 3,767,583 issued Oct. 23, 1973, to Fadgen and Saubestre also practice the use of the noble catalyst solution.
Processes of the above nature involving pretreatment and formation of a conductive film of copper or nickel for making the plastic conductive followed by electroplating have been commercially successful but have many technical and economic disadvantages.
From a technical point of view, intricate parts with blind-holes, threads, small diameter holes, slots, recesses or internal areas do not receive uniform electroplating because, by nature, certain plating baths have poor throwing power; for example, as in the case of chromium plating, and costly auxiliary anode assemblies, commonly employed in the industry must be used to plate into these extremely low current density areas. Excessive plated metal occurs on the high current density areas, such as sharp corners, protrusions, etc., while little or even no plating takes place in the low current density areas such as grooves, holes, slots, etc. In other words, uneven thickness of electrodeposits result due to the fact that the high current density areas receive more metal than the low current density areas.
Economically, the cost for applying an electroplated coating to the plastic is much greater because of the relatively thick deposits applied to the conductive film of copper or nickel. Excessive metal costs are incurred.
Furthermore, plastic parts, especially those large in size, require placement on plating racks for acceptable electroplating results. Fixture costs and labor for attaching the parts to said fixture bring about these additional process expenditures. Furthermore, unplated contact points appear where the parts are held on these fixtures which is highly objectionable in critical industrial applications.
There is a need for electroless plating compositions and processes producing bright deposits especially in the decoration of plastics which will overcome the aforementioned disadvantages while still being compatible with known commercial procedures. The present invention provides said compositions and processes.
While the application of bright electroless deposits, applied in bulk or mass handling, or with rack handling, is common practice when these deposits are applied to non-ferrous or ferrous substrates, the brightness can only be achieved by bright-dipping or polishing the metal substrate first to obtain these bright electroless finishes. Non-conductive surfaces are normally impossible to bright dip and polishing of non-conductive surfaces is not feasible since there would be a loss of adhesion of the electroless coating to the non-conductive surface due to the absence of mechanical or chemical interlocking of the metallic coating.
It is the principal object of the present invention to provide an effective and economical process for electrolessly applying attractive, bright metallic coatings to non-conductive surfaces, particularly a wide variety of synthetic resins, for example, acrylonitrile-butadiene-styrene (ABS), without the necessity of subsequent electroplating.
It is a further particular object of the present invention to provide a process whereby plastic parts, no matter how intricate in shape, form, or design, can receive a bright electroless coating uniformly deposited throughout the entire part without using costly racking and auxiliary anode assemblies to insure this uniform metal thickness in both high and low current density areas. In other words, the object is to provide a process wherein parts may be processed in bulk or in mass without the necessity of racking and still receive bright deposits of electroless metal with complete elimination of the need for electroplated deposits and related burdens of process and equipment.